If you're using electric motors with direct drive, the best option is usually all wheel independent drive. The extra motors and controllers are usually cheap compared to all the other stuff in a robot.For two wheel drive motors, you typically have the driving wheels being large, and in front, with one motor per driving wheel. The rear wheel or wheels are typically casters. Steering is done by driving the font wheels at different speeds - you can even pivot in place like this!Doing that kind of drive with casters in the front may lead to difficulty going straight, so front wheel drive is easier.